The present invention relates to a method for reducing emissions, such as sulfur oxides (SO.sub.2), nitrous oxides (N.sub.2 O and carbon monoxide (CO), to the atmosphere from the combustion of nitrogen containing combustible compounds. More specifically, this invention relates to a method for reducing such emissions when combusting solid fuels or the like in fluidized bed combustors.
Fluidized bed combustion (FBC) is known to result in efficient combustion, as well as, efficient sulfur oxide and nitrogen oxide emission control. Due to intimate mixing of solid material and gases an efficient combustion is achieved in the fluidized bed already at low combustion temperatures 700.degree.-1000.degree. C. Sulfur oxides (SO.sub.2) are captured at this relatively low combustion temperature, which is optimal for SO.sub.2 reduction by milled limestone injected with the fuel into the combustion chamber. The relatively low combustion temperature needed in a FBC also results in reduced formation of nitrogen oxides NO.sub.x, i.e. NO.sub.2 and NO. NO.sub.x emissions from FBC are typically in the range of 100-400 ppm.
The above mentioned improvements in fluidized bed technology over conventional flame combustion are enhanced in circulating fluidized bed combustion (CFBC). Besides providing the possibility of burning different fuels in the same combustor, i.e. both high and low grade fuels, the CFB boiler technology provides better means of controlling the combustion process leading to improved boiler efficiency and improved control of sulfur oxide (SO.sub.2) and nitrogen oxide (NO.sub.x) emissions. NO.sub.x emissions from CFB boilers are in the range of 150-200 ppm.
Recently attention has been focused on the emission of nitrous oxide (N.sub.2 O) from combustors. The atmospheric concentration of N.sub.2 O increases constantly and it is believed to have an effect on the atmosphere. While the greenhouse effect has mainly been associated with increased CO.sub.2 levels in the atmosphere, concern is now growing about strong infrared absorbers, such as N.sub.2 O, contributing to the greenhouse effect even if the concentration of N.sub.2 O is much lower than that of CO.sub.2. Further, according to recent research, N.sub.2 O may indirectly adversely affect the stratospheric ozone layer as well.
Recent studies indicate that fluidized bed combustion, while achieving significantly lower levels of NO.sub.x emissions compared to flame or pulverized coal combustion, may yield higher levels of N.sub.2 O. It has been reported that N.sub.2 O emissions are generated in higher degree in combustors with low combustion temperatures such as 750.degree.-900.degree. C. At higher temperatures the formation of N.sub.2 O does not seem to be a problem, as the formation of N.sub.2 O is minor, while the reduction of N.sub.2 O to N.sub.2 at the same temperature is high.
The likely main mechanism for N.sub.2 O formation from fuel nitrogen has been suggested to be the following: EQU FUEL-N.fwdarw.HCN EQU HCN+O.fwdarw.NCO EQU NCO+NO.fwdarw.N.sub.2 O
At the present time, however, the details of the mechanisms of N.sub.2 O formation are not known.
The combustion temperature and the type of fuel seem to be the main factors affecting the N.sub.2 O emission. According to tests the emissions decrease significantly when the combustion temperature is increased over 900.degree. C. In the combustion of coal, N.sub.2 O emissions varied typically from 30 to 120 ppmv (3% O.sub.2, dry), whereas in the combustion of oil shale, peat and wood waste N.sub.2 O emissions were typically significantly lower, below 50 ppmv.
There seems to be a strong correlation between temperature and both NO.sub.x and N.sub.2 O emissions. Changes to the combustion operating parameters affect NO.sub.x and N.sub.2 O emissions inversely. Increasing temperatures result in higher NO.sub.x and lower N.sub.2 O. Weaker correlations appear to exist for other parameters. A bed temperature increase in the combustion chamber would however result in reduced capability to capture SO.sub.2. Staged combustion seems to reduce both N.sub.2 O and NO.sub.x emissions to a certain degree, but easily leads to an increase in carbon monoxide (CO) concentration.
One method to reduce the N.sub.2 O emissions, suggested in U.S. Pat. No. 5,043,150, is to add hydrogen radicals to the flue gases by providing an additive capable of forming hydrogen radicals at temperatures equal to or higher than those of the flue gases. The hydrogen radicals effectively destroy N.sub.2 O through the homogenous gas reaction EQU (A) N.sub.2 O+H.fwdarw.N.sub.2 +OH
Additives providing hydrogen radicals are e.g. methane, liquified petroleum gas, oil, alcohol, pyrolyser gas, or gasifier gas. The hydrogen radical formation is favored at higher temperatures. Apparently by increasing the flue gas temperature the rate of the reaction (A) is also increased and a rapid N.sub.2 O destruction may be accomplished.
U.S. Pat. No. 5,048,432, European patent application EP 0 406 185, and German patent application DE 39 33 286 all suggest raising the temperature of flue gases to a level above 900.degree. C. for reducing N.sub.2 O emissions.
Other parameters potentially affecting N.sub.2 O emissions have also been studied, such as increase of excess air, injection of ammonia, recirculation of fly ash, CO concentration, and addition of limestone. Some studies show slight effects of above mentioned parameters, either decreasing or increasing N.sub.2 O emissions, but no clear picture has developed. E.g. N.sub.2 O has been found to decompose on the surface of calcined limestone CaO, while simultaneously the NO emissions increase. It has, on the other hand, also been reported that N.sub.2 O may result from NO reduction on CaSO.sub.x surfaces, CaSO.sub.x being formed by reduction of SO.sub.2 with CaO, the higher the Ca/S ratio the higher the NO reduction on CaSO.sub.4. Therefore, until now, no clear conclusion could be drawn on the effect of boiler limestone addition on emissions.
It is, however, known that N.sub.2 O emissions from fluidized bed boilers may be on the level of 50-200 ppm, i.e. higher than desired. Therefore, according to this invention a method is provided for reducing the emissions of N.sub.2 O from conventional fluidized bed boilers and circulating fluidized bed boilers, atmospheric or pressurized. The method according to the invention also may decompose CO in the flue gases, and improve the SO.sub.2 reduction in flue gases from a fluidized bed boiler.
The method of the invention simultaneously reduces N.sub.2 O, SO.sub.2, and CO in flue gases, thereby improving the environmental properties of fluidized bed combustor systems.